Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    12
    ReputationRep:
    why is the color change of phenolphthalein colorless to pink, and not the opposite? isnt it supposed to be neutral at the end point..
    Attached Images
     
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by pondsteps)
    why is the color change of phenolphthalein colorless to pink, and not the opposite? isnt it supposed to be neutral at the end point..
    Because it turns red at pH 9.7 ish, and adding one drop of NaOH to the acid just before neutralisation takes the pH to pH 10
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by charco)
    Because it turns red at pH 9.7 ish, and adding one drop of NaOH to the acid just before neutralisation takes the pH to pH 10
    but for methyl orange for eg. the end point is always orange, it doesnt take the color of the excess and change to yellow.. why does phenolphthalein take the color of the excess
    Offline

    14
    ReputationRep:
    (Original post by pondsteps)
    but for methyl orange for eg. the end point is always orange, it doesnt take the color of the excess and change to yellow.. why does phenolphthalein take the color of the excess
    We added an alkaline (NaOH) to an acid solution, the final pH would be higher than before, phenolphthalein is red/pink in alkaline conditions and colourless in acid conditions, therefore since the pH is higher than before, the colour change would be from colourless to pink/red

    same goes for methyl orange, it is orange/yellow in alkaline conditions and red in acidic conditions
    Offline

    17
    ReputationRep:
    If you add acid to alkali you get one colour change; if you add alkali to acid, you get the reverse colour change.

    We normally add acids to alkalis - it is usually easier to rinse acids out of burettes than it is alkalis.
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by PlayerBB)
    We added an alkaline (NaOH) to an acid solution, the final pH would be higher than before, phenolphthalein is red/pink in alkaline conditions and colourless in acid conditions, therefore since the pH is higher than before, the colour change would be from colourless to pink/red

    same goes for methyl orange, it is orange/yellow in alkaline conditions and red in acidic conditions
    so is the end point the neutral color or the excess of the reactant added(the titre)?
    Offline

    14
    ReputationRep:
    (Original post by pondsteps)
    so is the end point the neutral color or the excess of the reactant added(the titre)?
    The excess of titre added

    Posted from TSR Mobile
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by PlayerBB)
    The excess of titre added

    Posted from TSR Mobile
    then why is methyl orange always orange at the end point of the titration not red
    Offline

    14
    ReputationRep:
    (Original post by pondsteps)
    then why is methyl orange always orange at the end point of the titration not red
    No the colour change depend on the titre, on the reactant in the flask and on the pH change!!

    For methyl orange, it turns orange when an alkaline solution is added (titre), the end point is when the colour change occurs from red to orange, it occurs because if there was an acid the the flask and an alkaline was added from beurette then at the end point, all of the acid would have been neutralised and excess alkali would be found in the flask which causes the colour change to occur
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by PlayerBB)
    No the colour change depend on the titre, on the reactant in the flask and on the pH change!!

    For methyl orange, it turns orange when an alkaline solution is added (titre), the end point is when the colour change occurs from red to orange, it occurs because if there was an acid the the flask and an alkaline was added from beurette then at the end point, all of the acid would have been neutralised and excess alkali would be found in the flask which causes the colour change to occur
    sooo now the end point is yellow?? not orange.. bcz there's excess alkali
    Offline

    14
    ReputationRep:
    (Original post by pondsteps)
    sooo now the end point is yellow?? not orange.. bcz there's excess alkali
    You can say it's orange or yellow, it doesn't matter, the colour change occur from red to orange/yellow

    I suggest revising these indicators stuff from the book, I know it's misleading so maybe the book will have a better shot than I am
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by PlayerBB)
    You can say it's orange or yellow, it doesn't matter, the colour change occur from red to orange/yellow

    I suggest revising these indicators stuff from the book, I know it's misleading so maybe the book will have a better shot than I am
    aha, I got it anyway, thanks for helping out!!
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What's your favourite Christmas sweets?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.